1. Field of the Invention
The present invention relates to periodic focusing systems for guiding electron beams, and more particularly, to an alternative geometry for providing periodic focusing of an electron beam in a microwave amplification tube.
2. Description of Related Art
Microwave amplification tubes, such as traveling wave tubes (TWTs), are well known in the art. These microwave tubes, are provided to increase the gain, or amplify, an RF (radio frequency) signal in the microwave frequency range. A microwave RF signal induced into the tube interacts with an electron beam projected through the circuit. Energy within the beam thus transfers into the RF signal, causing the signal to be amplified.
Periodic focusing systems are well known in the art of microwave amplification tubes for guiding the electron beams which pass through beam tunnels within the microwave tubes. Focusing systems of this kind usually consist of ferro-magnetic material known as polepieces, having permanent magnets inserted between them. A microwave amplification tube can either utilize an "integral polepiece" or a "slip-on polepiece." An integral polepiece forms part of a vacuum envelope extending inward towards the beam region, while a slip-on polepiece lies completely outside the vacuum envelope of the tube. The magnets are typically ring-like so as to completely surround the tube or can be button shaped so as to cover azimuthally only portions of the inter-polepiece region. In all cases, however, the tube geometry as dictated by the focusing system is essentially cylindrical.
Examples of prior art cylindrical geometry periodic permanent magnet (PPM) focusing systems are shown in FIGS. 1-3. The tubes incorporating prior art PPM focusing systems comprise a plurality of substantially annular polepieces 12 which alternate with non-magnetic spacers 14. The polepieces 12 are commonly formed from iron, while the non-magnetic spacers 14 are typically formed from copper. The polepieces 12 extend radially outward relative the tubes, having ends 15 which join to permanent ring magnets 16 and hubs 13 which form a portion of an electron beam tunnel 17. The polepieces 12 may also be hubless, in which they resemble washers. The circuit tube elements are symmetrical, forming the cylindrical shape shown in FIG. 2, with the electron beam tunnel 17 extending through its center. The configuration of FIG. 1 is known as a single period focusing system, since the polarity of each of the permanent magnets 16 reverses with each adjacent pair of polepieces 12. An alternative configuration is shown in FIG. 3, which discloses a double period focusing system. Interspersed between the polepieces 12 are intermediate polepieces 18. The permanent magnets 16 join each adjacent pair of polepieces 12, spanning two adjacent non-magnetic spacers 14 and an intermediate polepiece 18.
In each of these cylindrical geometry PPM focusing systems, the magnetic flux that enters the polepiece 12 at the boundary with the magnet 16 is first transported radially inward. Magnetic flux that reaches the beam tunnel 17 at an inner radius of the polepiece 12 then jumps axially to its neighboring polepieces, thereby linking the beam tunnel region with a magnetic field to focus the beam. The flux direction inside the polepiece 12 is essentially radial (R) and axial (Z). Accordingly, such cylindrical geometry focusing systems can be referred to as R-Z PPM focusing systems.
These R-Z PPM focusing systems have a desirable feature in that the flux is concentrated at the inside diameter of the polepiece 12, which is often near the region where the beam must be focused. However, these systems also have an inherent limitation which results from the radial length of the circular geometry. In a traveling wave tube which utilizes the R-Z PPM focusing system, an RF path for the microwave signal is provided through the tube. For example, a coupled cavity traveling wave tube would include numerous tuned cavities which determine the bandwidth of the amplified RF signal. The diameter of the ring magnet which surrounds the tube would thus be limited by the required cavity size within the tube. However, as the diameter of the ring magnet system increases to accommodate larger cavities, or the azimuthal position of the pill magnet extends radially outward, the magnetic field strength concentrated in the beam tunnel would decrease. In microwave amplification tubes using high perveance electron guns, the magnetic field strength may be too weak to adequately focus the electron beam.
A related problem with circular geometry PPM focusing systems is that of heat removal. As the electron beam drifts through the beam tunnel 17, heat energy resulting from stray electrons intercepting the tunnel walls must be removed from the tube to prevent reluctance changes in the magnetic material, thermal deformation of the cavity surfaces, or melting of the tunnel wall. Typically, the heat must flow outwardly from the tunnel wall, through the polepieces 12 to a point outside the tube where one or more heat sinks can draw the heat out of the tube. The copper spacers 14 also conduct the heat away from the beam tunnel 17. As with the magnetic flux conduction problem described above, large diameter tubes have a more difficult heat conduction problem in that the heat has further to travel before reaching an external heat sink. Reducing the diameter of the tube would allow the heat to be removed more readily, but would not be compatible with tubes having larger sized coupled cavities.
Consequently, the prior art focusing system forces microwave tube designers to sacrifice both magnetic flux density and thermal ruggedness in order to allow an internal RF path. Thus, it would be desirable to provide a periodic focusing system for a microwave amplification tube which permits either lessening of the thermal resistance of the thermal path from the tunnel wall to the heat sink, or increasing the magnetic flux level at the beam tunnel region, while maintaining a portion of the tube adjacent the tunnel for the RF path or other uses.